Carbene-Dichlorosilylene Stabilized Phosphinidenes Exhibiting Strong Intramolecular Charge Transfer Transition

The
unstable species dichlorosilylene was previously stabilized
by carbene. The lone pair of electrons on the silicon atom of (carbene)­SiCl2 can form a coordinate bond with metal–carbonyls. Herein
we report that (carbene)­SiCl2 can stabilize a phosphinidene
(Ar–P, a carbone analogue) with the general formula carbene→SiCl2→P–Ar (carbene = cyclic alkyl­(amino) carbene
(cAAC; 2) and N-heterocyclic carbene (NHC; 3)). Compounds 2 and 3 are stable, isolable,
and storable at 0 °C (2) to room temperature (3) under an inert atmosphere. The crystals of 2 and 3 are dark blue and red, respectively. The intense
blue color of 2 arises due to the strong intramolecular
charge transfer (ICT) transition from πSiP→π*cAAC. The electronic structure and bonding
of 2, 3 were studied by theoretical calculations.
The HOMO of the molecule is located on the πSiP bond, while the LUMO is located at the carbene moiety (cAAC or NHC).
The dramatic change in color of these compounds from red (3, NHC) to blue (2, cAAC) is ascribed to the difference
in energy of the LUMO within the carbenes (cAAC/NHC) due to a lower
lying LUMO of cAAC.